1. Field of the Invention
The present invention relates to a driving device for a vehicle, which is provided with an input member that is connected in a driving manner to an internal combustion engine, an output member that is connected in a driving manner to wheels, a first rotating electrical machine, a second rotating electrical machine, a differential gear unit including at least three rotational elements, and a control device.
2. Description of the Related Art
For example, a technology disclosed in JP-A-2010-76678 may be exemplified as a driving device for a vehicle as described above in the related art. JP-A-2010-76678 discloses a configuration in which the differential gear unit includes a planetary gear mechanism having three rotational elements, the first rotating electrical machine is connected in a driving manner to a sun gear, the input member is connected in a driving manner to a carrier, and the second rotating electrical machine and the output member are connected in a driving manner to a ring gear. In addition, the driving device for a vehicle includes a frictional engagement device that is capable of releasing the connection in a driving manner between the carrier and the input member (internal combustion engine), such that the internal combustion engine is detachable during executing an electric traveling mode in which a vehicle is made to travel by torque of the second rotating electrical machine in a state in which the internal combustion engine is stopped. Therefore, during executing the electric traveling mode, the rotation speed of the sun gear (the first rotating electrical machine) or the carrier may be set independently from a vehicle speed. As a result, for example, as disclosed in JP-A-2010-76678, the carrier is made to rotate by actively controlling the rotation speed of the first rotating electrical machine, and auxiliary machines may be driven using the rotation of the carrier.
In the configuration disclosed in JP-A-2010-76678, when the electric traveling mode is switched to a split traveling mode in which the torque of the internal combustion engine is used to drive, the rotation speed of the internal combustion engine is made to increase to a rotation speed at which ignition is possible by torque of the first rotating electrical machine. Specifically, as disclosed in paragraphs 0073 to 0076 of JP-A-2010-76678, the rotation speed of the carrier is decreased by decreasing the rotation speed of the first rotating electrical machine, and in a state where the rotation speed of the carrier becomes equal to zero that is a rotation speed of the internal combustion engine (in a state indicated by a bold broken line in FIG. 9 of the literature), the frictional engagement device is switched from a release state to a direct engagement state. Then, the rotation speed of the internal combustion engine is increased by the torque of the first rotating electrical machine, and the internal combustion engine is started up by the ignition in a state in which the rotation speed of the internal combustion engine is equal to or higher than the rotation speed at which the ignition is possible (in a state indicated by a bold solid line in FIG. 9). In this manner, the transition from the electric traveling mode to the split traveling mode may be realized.
However, as is clear from FIG. 9, the rotation speed of the first rotating electrical machine, in a state in which the rotation speed of the carrier and the rotation speed of the internal combustion engine become equal to each other, is determined in response to the vehicle speed. That is, the higher the vehicle speed, the larger an absolute value of the rotation speed of the first rotating electrical machine in the state. In addition, the magnitude of the maximum torque, which may be generated by the rotating electrical machine, is apt to be small, as the absolute value of the rotation speed becomes large. Therefore, in the configuration of JP-A-2010-76678, the torque of the first rotating electrical machine may be deficient depending on the magnitude of the vehicle speed with respect to the torque necessary to increase the rotation speed of the internal combustion engine, such that there is a concern in that the internal combustion engine may not be started up, or vibration may occur at the time of starting up the internal combustion engine.
In addition, in the paragraphs 0067 to 0068 of JP-A-2010-76678, there is disclosed a configuration in which at the time of the switching from the electric traveling mode to the split traveling mode, the engagement of the frictional engagement device is made while being slid so as to be switched into a direct engagement state in a state in which the rotation speed of the first rotating electrical machine is not decreased and a difference in the rotation speed between the carrier and the internal combustion engine is relatively large. However, as is clear from FIG. 9, since the higher the vehicle speed, the larger the difference between the rotation speed of the carrier and the rotation speed (zero) of the internal combustion engine, the difference in the rotation speed becomes too large depending on the magnitude of the vehicle speed, such that there is a concern in that an increase in the temperature of the frictional engagement device becomes too high, or the time necessary for the engagement is too long.